1. Field of the Invention
The present invention relates to a method of forming an outer ring of a constant velocity joint which outer ring comprises a shaft and a cup formed integral with and expanding radially and outwardly of one end of the shaft.
2. Description of the Related Art
In the power transmission of an automobile, for example, there is employed a constant velocity joint in order that a smooth torque may be obtained without being affected by the angle of orientation of the shaft to be driven. As constant velocity joints of this kind, there are known a Birfield type joint which transmits a torque by means of a ball bearing and a tripod type joint which transmits a torque through at least three rollers.
In the above case, each of the Birfield type and tripod type constant velocity joints is provided with an outer ring (outer member) comprising a shaft and a cup formed integral with the shaft and extending radially and outwardly of one end of the shaft and having a plurality of track grooves on the inner peripheral surface thereof.
Where, for example, the outer ring which forms part of the Birfield type constant velocity joint is manufactured, a rod-shaped material 2 is prepared as shown in FIG. 8A and a molded body 6 having a shaft portion 4 and a solid body portion 2a is obtained by subjecting the material 2 to forward extrusion molding (refer to FIG. 8B). Next, as shown in FIG. 8C, by applying an upsetting process to the molded body 6, the solid body portion 2a is extruded to form an upset portion 8 and then the upset portion 8 is extrusion-molded to thereby form a cup portion 10 (refer to FIG. 8D). Further, as shown In FIG. 8E, an ironing (sizing) process is applied to the inner peripheral surface of the cup portion 10 so that an outer ring member 14 as a product having a plurality of track grooves 12 is obtained.
In the above described forming process, the shaft portion 4 is set to have a predetermined degree of product accuracy by being extrusion-molded to a comparatively large length In the axial direction through the forward molding process shown in FIG. 8B. On the other and, the cup portion 10 is formed by a plurality of forming processes including upset molding, backward extrusion molding and ironing processes as shown in FIGS. 8C through 8E.
Thus, the processes for forming the shaft portion 4 and the cup portion 10 are performed separately from one another and particularly, the cup portion 10 is formed by a plurality of molding processes so that the lowering of the level of accuracy due to a die accuracy error or the abrasion of the die in each of the processes tends to take place. Consequently, the problem Is being pointed out that the accuracy of concentricity between the shaft portion 4 and the cup portion 10 formed In advance by the forward extrusion molding can not be secured and processes for obtaining the roundness of the inner peripheral surface of the cup portion 10 and the concentricity of the cup with the shaft 4 become necessary before grinding the track grooves 12 and the inner peripheral surface of the cup portion 10, which results in requiring many man-hours. Further, a problem also arises that many man-hours are required to perform a grinding process for securing the product accuracy with respect to the track grooves 12 and the inner spherical surface of the cup portion 10.